DOC: fix a few typos

include/types/proto_http.h: hwen -> when
include/types/server.h: SRV_ST_DOWN -> SRV_ST_STOPPED
src/backend.c: prefer-current-server -> prefer-last-server

Signed-off-by: Godbach <nylzhaowei@gmail.com>
diff --git a/include/types/proto_http.h b/include/types/proto_http.h
index 95bf59d..2654b78 100644
--- a/include/types/proto_http.h
+++ b/include/types/proto_http.h
@@ -71,7 +71,7 @@
 
 /* indicate how we *want* the connection to behave, regardless of what is in
  * the headers. We have 4 possible values right now :
- * - WANT_KAL : try to maintain keep-alive (default hwen nothing configured)
+ * - WANT_KAL : try to maintain keep-alive (default when nothing configured)
  * - WANT_TUN : will be a tunnel (CONNECT).
  * - WANT_SCL : enforce close on the server side
  * - WANT_CLO : enforce close on both sides